(Important note: This is not the cute and paradoxically well-known 'secret menu' you've heard about that gives one access to extra beef-patties and such: this is the True Secret Menu, known only to clientele that have consumed their way to the Fourth Level. I reveal the secrets of the True Secret Menu at great personal peril, so that at last the public can be warned of the dark eldritch mysteries of this privately owned fast-food enterprise.)

The menu, which is scarred onto the back of every employee, is as follows:

THE SUPPLICANT (AVAILABLE 9:00AM TO 1:00PM)

Approach the employee of your choosing on your knees (preferably you have pilgrimaged to In-Und-Out in such prostration). The employee will place a single leaf of lettuce upon your head. Darkness will fall over you.
When you regain consciousness, you will be in the care of several young, virginal female staff members. They will gently knead you while singing 'Bringing in the Sheaves' (backwards). Your skin will become loose and pliant, to the point that you can be rolled and flattened into fresh patties, and lovingly seasoned and prepared (never frozen). You will then be voraciously consumed by the aforementioned virgins, and later daintily excreted into the sewer. There you will join the others.

LOT'S WIFE

You must enter from the employee access door. You will be immediately swarmed by screaming, knife wielding staff-members demanding your intentions. Tell them you want extra-crunchy fries, and stoicly await your bludgeoning.
When you awake, you will be in an unfamiliar room with no obvious exit. You will be naked and lightly salted.
High quality vegetable oil will begin to pour down from spouts in the ceiling. Panels in the wall will slide away to reveal triple-layer, high tempered windows. On the opposite side you will see a congregation of employees gazing reverently as the room inexorably fills. Any employee that you directly plead with will receive a gift certificate to Barnes and Noble, as well as a special mention in the company newsletter.
At the point that your struggles diminish with exhaustion, the floor will begin to heat up.
If you are even slightly overdone, the sun will implode.

THE FALSE PROPHET
Approach the counter and order menu-items from a leading competitor. When you are gently informed by the smiling, attractive cashier that what you wish to have is such, you must become belligerent. Do not accept any compromise that is offered, even a compensatory coupon.
At this point all doors will automatically seal tight and triple-lock. Steel shades will drop across the windows. You and whomever else currently patronizing the restaurant in question will be trapped.
The cashier will type "YHVH" into the register, and the main floor will drop away. All will plummet into The Processing Room of Blasphemers, where you and the hapless patrons present for your deed will be thoroughly ground together, and then compressed into small nuggets, which will then be urinated on by the evening-shift (urine is very purifying, especially that of the employees of In-Und-Out).

This human melange will finally be left at the rear entrance of a competitor. Undoubtedly the Heathens will add this cursed bounty to their filthy, frozen supplies.

THE APOSTLE
As a prerequisite for this order, you must be a virgin, male or female. You must be clean, which is to say, you must have never indulged in the fare of another fast-food establishment. You must be sure of this, for the taint of inferior product will be recognized.
You must divest yourself of all your worldly possessions by the time you enter one of the many available In-Und-Out eateries in your area. In your hand should be a hamburger you have made yourself; the bun should be personally milled and baked, and the meat patty should be that of a cow that you alone have slaughtered and prepared. This should be in a nice white bag with the In-Und-Out label carefully hand-etched onto it.
The cashier will accept this offering (if properly prepared), and will then stare at you silently for ten minutes. You must meet his/her gaze, and you must not blink. You should also have a pleasant, attractive smile on your face. Do not let it waver for an instant!
The cashier will hit the Holy Initiate Key on the register. You will fall through a trapdoor. All patrons witness to this will be given a complimentary meal for their silence, and later killed in their homes.
You will find yourself in a small windowless room with a desk. On this desk will be a pen and an application form. Fill it out.
You will be informed of your acceptance or rejection within two weeks, by way of special courier or holy assassin, respectively.

THE IMMACULATE DELUXE (SUNDAY ONLY)
You will order this by name, at the counter. You will be given a regular hamburger, at the usual price (cheese extra). You will take this hamburger to the outside rear of the restaurant. After you eat it, you will fall unconscious.
You will awake in a very small cloister. You will be very far from all you know.
Through a slot in your locked door, you will be given daily allotments of bread and water, as well as an In-Und-Out drink cup (empty).
Your cloister will contain a quill pen, a bucket of ink, and a special In-Und-Out translation of The Holy Bible.
You are to spend your days memorizing scripture, and transcribing appropriate selections onto the bottoms of drink cups, in perfect script.
Your mistakes must be minimal. You are to flagellate yourself with a steel spatula for every wasted cup.
After seven years of unquestioning service-with-a-smile, a hamburger will be presented to you in lieu of your bread scrap.
This will be The Immaculate Deluxe. It will be the best hamburger you have ever had, with of course only the freshest ingredients, never frozen.
After you finish your meal, a servant will slit your throat. You will become a being of light, and you will rise up to God, who will then eat you.

THE GIFT OF THE MAGI
Simply ask for a free hamburger. Make this request without emotion or inflection. You will be immediately presented with a hamburger.
Do not eat your meal at the restaurant. You must take it home.
Once there, immediately begin consumption. You will discover that there is a large, dead locust in the center of the patty. Continue eating.
Failing this, you will likely be inclined to call the authorities or the media in the hope of retribution or a large cash settlement. If this attempt is made, you will find that your phone has been redirected. The voice on the other end will tell you to look in your refrigerator.
There you will find your own dismembered corpse.
In either case: You are part of the In-Und-Out family now.